Is /Volumes/DiskStation actually a volume, or just a regular folder that happens to be placed in /Volumes? If it's a regular folder, my guess would be that you have some program running (possibly as a background process) that's trying to access something in /Volumes/DiskStation; if the NAS were mounted there, it would access its file(s) off the NAS as it's (presumably) supposed to, but if the NAS isn't mounted under that name a new folder gets created there... which prevents the NAS from mounting under that name, thus perpetuating the problem.
When /Volumes/DiskStation gets recreated, is anything created inside it? If so, that's a good indication that this is what's going on, and what gets created may point to the culprit.
If this is the problem, you need to find & shut down whatever program is creating /Volumes/DiskStation, then unmount the NAS (removing /Volumes/DiskStation-1) and remount it (this time /Volumes/DiskStation should be available, and it should use that name). Then you can restart the program that's using it.
To keep the problem from reappearing, you need to avoid ever launching that program when the NAS isn't already mounted.